There are methods for deciphering control words for a first terminal and at least one second terminal that are mechanically and electronically independent of one another, wherein:                the first and second terminals respectively transmit cryptograms CW*1,t and CW*2,t to a same control-word server,        in response the control-word server deciphers the cryptograms CW*1,t and CW*2,t to obtain respectively control words CW1,t and CW2,t, the control words CW1,t and CW2,t enabling the descrambling, respectively, of first and second multimedia contents broadcast simultaneously on first and second channels respectively, and then,        the control-word server transmits the control words CW1,t and CW2,t, respectively to the first and second terminals.        
The term “multimedia content” designates an audio and/or visual content designed to be rendered in a form directly perceptible and comprehensible to a human being. Typically, a multimedia content corresponds to a succession of images forming a film, a television show or advertising material. A multimedia content can also be an interactive content such as a game.
There are known ways of broadcasting several multimedia contents at the same time. To this end, each item of multimedia content is broadcast on its own channel. The channel used to transmit a multimedia content is also known as a “station”. A channel typically corresponds to a television station. This enables the user to choose simply the multimedia content that he wishes to view in changing channels.
To secure and subject the viewing of multimedia contents to certain conditions, such as the payment of a subscription for example, the multimedia contents are broadcast in scrambled form and not in plain or unencrypted form. More specifically, each multimedia content is divided into a sequence of cryptoperiods. Throughout the duration of a cryptoperiod, the conditions of access to the scrambled multimedia content remain unchanged. In particular, throughout the duration of a cryptoperiod, the multimedia content is scrambled with the same control word. Generally, the control word varies from one cryptoperiod to another. Furthermore, the control ward is generally specific to a multimedia content. Thus if, at a given instant, N multimedia contents are broadcast simultaneously on N channels, then there are N different control words each used to scramble one of these multimedia contents.
Here, the terms “scramble” and “encipher” are considered to be synonyms.
The plain or unencrypted multimedia content corresponds to the multimedia content before it is scrambled. This content can be made directly comprehensible to a human being without recourse to descrambling operations and without placing conditions on the viewing of this content.
The control words needed to descramble the multimedia contents are transmitted synchronizedly with the multimedia contents. For example, the control words needed to descramble the (t+1)th cryptoperiod are received by each terminal during the tth cryptoperiod. To this end, for example, the control words are multiplexed with the scrambled multimedia content.
To secure the transmission of the control words, these words are transmitted to the terminals in the form of cryptograms. The term “cryptogram” herein designates a piece of information that is not sufficient by itself to retrieve the control word in plain form. Thus, if the transmission of the control word is intercepted, knowledge of the control word cryptogram alone does not make it possible to retrieve the control word by which the multimedia content can be descrambled. To retrieve the plain control word, i.e. the control word used to directly descramble the multimedia content, it must be combined with a piece of secret information. For example, the cryptogram of the control word is obtained by enciphering the plain control word with a cryptographic key. In this case, the secret information is the cryptographic key used to decipher this cryptogram. The cryptogram of the control word can also be a reference to a control word stored in a table containing a multitude of possible control words. In this case, the secret information is the table associating a plain control word with each reference.
The secret information should be kept in a secure place. To this end, it has already been proposed to store the secret information:                either in security processors such as hip cards directly connected to each of the terminals,        or more recently in control-word servers common to several terminals.        
In the latter case, the terminals are devoid of chip cards. These terminals are then called cardless terminals.
The control-word server is connected to each of the terminals by a long-distance information-transmission network such as the Internet. When a control-word server is used, the cryptograms of the control words are first of all transmitted to the different terminals and then forwarded by these terminals to the control-word server. This procedure has several advantages. In particular, the information-transmission network used to broadcast the multimedia contents and the cryptograms of the control words can be different from the one used to connect the terminals to the control-word server. For example, the network for broadcasting multimedia content and cryptograms of the control words is a one-way network with a large bandwidth, for example a satellite network. Conversely, the network connecting the terminals to the control-word server is a two-way network with a bandwidth that may be smaller.
Then, this simplifies the time synchronization between the broadcasting of the multimedia contents and the broadcasting of the cryptograms of the corresponding control words.
The control-word server has the function of deciphering the cryptograms of the control words transmitted by the terminals and then sending the deciphered control word back to each of these terminals. Thus, in a way, the control-word server plays the role of a chip card common to several terminals that are mechanically and electrically independent of one another. Terminals that are electronically independent of one another are terminals that can work autonomously and have no electronic part common to them,
When a terminal needs a control word to descramble a multimedia content, it sends the control-word server a request containing the cryptogram of the control word. In response, the control-word server deciphers this cryptogram and then sends the deciphered control word to the terminal which can then descramble the desired multimedia content.
The multimedia contents broadcast over the different channels are temporally coordinated with one another. For example, the multimedia content broadcasting times are set so as to comply with the broadcasting times indicated in a pre-established program schedule. Each terminal on a given channel therefore receives appreciably the same multimedia content at the same time.
Thus, it often happens that users change channels (or stations) at the same time. For example, a simultaneous change of channel of this kind can be prompted by the broadcasting of an advertising sequence on the channel being currently viewed. The user is then said to have “zapped channels”.
In response to this change of channel, each terminal immediately transmits a request to the control-word server and, in response, receives the control word needed to descramble of the multimedia content currently being broadcast on the new channel being viewed. Thus, a massive and simultaneous change from one channel to another will cause a peak in the workload of the control-word server.
The computation power of the control-word server depends on this load peak. Thus, the greater the load peak, i.e. the greater the maximum number of requests to be processed in a predetermined time span, the greater is the computation power needed by the control-word server.
These workload peaks should be reduced as far as possible in order to limit the computation power needed by the control-word server while at the same limiting modifications made to the scrambled multimedia content transmission system.